Emissions of mercury from coal-fired and oil-fired power plants have become a major environmental concern. Mercury (Hg) is a potent neurotoxin that affects human health at very low concentrations. The largest source of mercury emission in the United States is coal-fired electric power plants. These coal-fired power plants account for between one-third and one-half of total mercury emissions in the United States.
The mercury emission is predominantly through the flue gas (exhaust gas) ejected from the burning coal. There are three basic forms of Hg in the flue gas: elemental Hg; oxidized Hg; and particle-bound mercury.
Currently, the most common method for mercury emissions reduction from coal-fired and oil-fired power plants is the injection of powdered activated carbon into the flue stream. The activated carbon provides a high surface area material for the adsorption of the mercury and the agglomeration of the particle bound mercury. The disadvantage of adding activated carbon into the flue stream is the retention of the activated carbon in the fly ash waste stream. Fly ash from coal-fired power plants if often added to concrete, where the presence of the activated carbon adversely affects the performance.
Another method for reducing Hg emissions is through the addition of chemical species that react with mercury to chem-adsorb the elemental Hg and oxidized Hg. One class of materials capable of chemically reacting with Hg are metal sulfides. U.S. Pat. No. 6,719,828 teaches the preparation of layered sorbents such as clays with metal sulfide between the clay layers. The method used to prepare the layered sorbents is based on an ion exchange process, which limits the selection of substrates to only those having high ion exchange capacity. In addition, the disclosed ion exchange is time-consuming, involving several wet process steps significantly impairing the reproducibility, performance, scalability, equipment requirements, and cost of the sorbent. The process of making a sorbent, in accordance with the teachings of U.S. Pat. No. 6,719,828, involves swelling a clay in an acidified solution, introducing a metal salt solution to exchange metal ions between the layers of the clay, filtering the ion exchanged clay, redispersing the clay in solution, sulfidating the clay by adding a sulfide solution, and finally filtering and drying the material. Another shortcoming of the process disclosed in U.S. Pat. No. 6,719,828 the environmental liability of the by-products of the ion exchange process, i.e., the waste solutions of metal ions and the generated hydrogen sulfide.
Published U.S. patent application Ser. No. 11/291,091 teaches the preparation of metal•sulfide/bentonite composites for the removal of mercury from flue gas streams. The application teaches two methods, an incipient wetness process and a solid-state reactive grinding process, to prepare the composites. The processes are similar in that a copper salt is mixed with a bentonite clay and then a sulfide salt is added. The processes differ in the method of addition of the sulfide salt. In the first method the sulfide salt is added through an “incipient wetness” procedure where the sulfide salt is dissolved in water and added to the copper/clay mixture as an aqueous solution; in the second method the sulfide salt is added through a “solid-state reactive grinding” process where the sulfide salt hydrate is ground with the hydrated copper/clay mixture. The application further teaches that the incipient wetness and solid-state grinding methods differ from the “wet” method of U.S. Pat. No. 6,719,828 because there is no ion-exchange of the copper ion for the cationic ions of the bentonite clay. The composite nature of the materials produced in the application are supported by powder X-ray diffraction spectra that provide evidence of the formation of covellite (CuS), the same copper sulfide prepared in U.S. Pat. No. 6,719,828.
While U.S. application Ser. No. 11/291,091 disclaims ion exchange, copper salts and bentonite clays readily and easily ion exchange to yield very stable copper/clay compositions. See e.g., Ding, Z. and R. L. Frost, “Thermal study of copper adsorption on montmorillonites” Thermochimica Acta, 2004, 416, 11-16. Analytical analysis of these compositions confirms both interlayer ion-exchange (intercalation) and edge adsorption of the copper salt. See e.g., El-Batouti et al. “Kinetics and thermodynamics studies of copper exchange on Na-montmorillonite clay mineral” J. Colloid and Interface Sci. 2003, 259, 223-227.
There is still an ongoing need to provide improved pollution control sorbents and methods of their manufacture. It would be desirable to provide sorbents containing metal sulfides on substrates that can be easily and inexpensively manufactured. In this regard, simple and environmentally friendly methods that effectively convert readily available substrates to chemical sorbents, which do not require the numerous steps involved in an ion exchange process, are needed.